1. Field of the Invention
The present invention relates to a method for crystallizing 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,903,11]-dodecane, hereinafter referred to and known in the art as CL-20 and HNIW.
2. Description of the Related Art
For many existing propellant and weapons systems, a critical ingredient for enhancing propulsive and explosive performance is the energetic filler. CL-20, with its substantial increase in performance output over most energetic fillers, presents significant opportunities in terms of energy capabilities for propellants and explosives. For example, the use of CL-20 as the energetic filler or propellant component in weapons systems may provide increased anti-armor penetration, enhanced missile payload velocity and flight, increased underwater torpedo effectiveness and lethality, and improved gun propellant impetus.
The performance of CL-20 in propellant and weapon systems is highly dependent upon the crystal polymorph of CL-20. CL-20 may undertake several different crystal polymorphs, the most preferred of which is a high density phase known in the art and referred to herein as the ε-polymorph (or epsilon-polymorph) of CL-20. The ε-polymorph of CL-20 is preferred because of the high energetic performance and density, and lower sensitivity compared to other polymorphs. However, many conventional CL-20 synthesis techniques produce non-epsilon polymorphs, especially α-polymorph, in relatively large amounts. The α-polymorph has a much lower density than the ε-polymorph, and, therefore, is less desirable for use in propellant weapon systems. For these reasons, CL-20 synthesized by many conventional techniques must be dissolved and subjected to re-crystallization in order to increase the yield of the ε-polymorph to acceptable levels.
A CL-20 crystallization technique is disclosed in U.S. Pat. No. 5,874,574 to Johnston et al., which describes a process by which CL-20 is precipitated into its epsilon polymorph. According to an aspect of this technique, CL-20 is dissolved in a solution containing a non-chlorinated CL-20 solvent, such as ethyl acetate. The CL-20 solvent solution is dried, and a low density non-chlorinated CL-20 non-solvent is then added to the dry CL-20 solvent phase to cause precipitation of E-polymorph CL-20 crystals. Non-solvents include aromatics, such as benzene and toluene and the like, and relatively lower carbon number hydrocarbons.
The technique of the Johnston et al. patent is particularly effective over most conventional methods in crystallizing ε-polymorph CL-20 prepared from its TADF (tetraacetyldiformylhexaazaisowurtzitane) precursor. However, application of the same crystallization technique to CL-20 prepared from its TADA (2,6,8,12-tetraacetyl-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,903,11]-dodecane or “TADH”) precursor has certain drawbacks. In particular, addition of the non-solvent to the dry CL-20 solvent solution causes precipitating CL-20 crystals to stick to the crystallizer (e.g., container, beaker, or tank) in which the crystallization is conducted. In some instances, as much as 10 to 20 weight percent of the CL-20 crystal yield remains stuck to the crystallizer walls. In order to remove the precipitated CL-20 crystals from the crystallizer, the CL-20 crystals are redissolved into solution with a CL-20 solvent, such as ethyl acetate, then are recrystallized with a non-solvent. With each recrystallization, a smaller amount of precipitate sticks to the crystallizer walls. Often, however, this process must be repeated several times to produce a high yield without leaving appreciable amounts of CL-20 stuck to the crystallizer. In addition, the crystals form as unusable large agglomerates because of the inability to grow on all surfaces of the crystal.
Evaporation is another known CL-20 crystallization technique. The evaporation technique involves preparing a saturated solution of solvent and non-solvent, seeding the saturated solution with CL-20 crystal seeds, and evaporating the solvent. The solvent is removed slowly by evaporation, leaving CL-20 crystals in the non-solvent. A drawback to the evaporation technique is its expense and difficulties involved with lot-to-lot (or batch-to-batch) reproducibility. Variance in CL-20 particle size distribution and quality from batch to batch often demands the practice of post-crystallization grinding. However, grinding adds to production costs. Also, grinding of energetic materials may raise safety rated risks.
It continues to be a desirability in the art to provide a CL-20 crystallization method that is inexpensive yet produces CL-20 particles of reproducible crystal size without requiring recrystallization of particles stuck in the crystallizer or post-crystallization grinding operations.